Im thinkin about getting into dart frogs,green and black mantellas,bumble bee darts and blue dart frogs have caught my eye. Can you mix darts or is it strictly one type? Also i only want to a 20L tank or something of that size(im running out of room!) So how many can go in a tank roughly that size?I also read if you take care of these guys and give them proper breeding conditions they breed like crazy......what do you all recommend? im open for all suggestions

Mixing darts is highly frowned upon, in addition to the usual stresses and difficulties that may occur, many dart frogs are also capable of cross breeding, and hybrid dart frogs are not something we want in the hobby. A 20L is sufficient for most darts, if you want to keep the blues (azureus) or other tinctorius morphs then you should get a sexed pair, or raise a group of 4-6 juveniles and be prepared to split them up or find new homes for the ones you don't want when they reach maturity, in a tank that small females will fight, sometimes to the death. Bumble bees (leucomelas) are more sociable, you could keep a group of four in a 20L comfortably. Check out dendroboard.com, lots of good information there.